Quantum-well semiconductor devices can be configured to detect radiation with improved performance compared to many other types of radiation detectors. Unique properties of the quantum-well structures allow for a high quantum efficiency, a low dark current, compact size and other advantages.
In particular, various quantum-well structures can be formed by artificially varying the compositions of lattice matched semiconductor materials to cover a wide range of wavelengths in infrared ("IR") detection and sensing. An intraband transition, that is, photoexcitation of a carrier (i.e., an electron or a hole) between a ground state and an excited state in the same band (i.e., a conduction band or a valance band), can be advantageously used to detect radiation with a high responsivity in the IR range at a selected wavelength by using a proper quantum-well structure biased at a proper voltage. For example, the absorption wavelength of a quantum-well structure formed of Al.sub.x Ga.sub.1-x As/GaAs can be changed by altering the molar ratio x (0.ltoreq.x.ltoreq.1) of aluminum or the thickness of GaAs layer. Other materials for infrared detection include Hg.sub.1-x Cd.sub.x Te and Pb.sub.1-x Sn.sub.x Te. See, for example, Gunapala and Bandara, "Recent Developments in Quantum-Well Infrared Photodetectors," Physics of Thin Films, Vol. 21, pp. 113-237, Academic Press (1995) and a commonly assigned pending Application Ser. No. 08/785,350 filed on Jan. 17, 1997, which are incorporated herein by reference.
Infrared sensing arrays formed of quantum-well structures are desirable due to their applications in night vision, navigation, flight control, environmental monitoring (e.g., pollutants in atmosphere) and other fields. Many conventional infrared arrays respond to radiation only in a specified wavelength range, such as a short-wavelength infrared range ("SMIR") from about 1 to about 3 .mu.m, a mid-wavelength infrared range ("MWIR") from about 3 to about 5 .mu.m, a long-wavelength infrared range ("LWIR") from about 8 to about 12 .mu.m, or a very-long-wavelength infrared range ("VLWIR") that is greater than about 12 .mu.m. All sensing pixels in a quantum-well sensing array operating at a specified radiation wavelength are biased at a predetermined voltage. A readout multiplexer having an array of readout pixels corresponding to sensing pixels is usually used to supply this common bias voltage and to read out the signals from the sensing pixels.
A sensing array may have sensing pixels that each include a MWIR detector and a LWIR detector to form a dual-band array. Hence, simultaneous detection of radiation signals can be achieved at two different IR ranges in the same array.
Several dual-color single-element quantum-well detectors have been proposed. Two quantum-well detectors for two different wavelengths can be stacked together to form a single detector for detecting two radiation at two different wavelengths. See, for example, Tidow et al., "A High Strain Two-Stack Two-Color Quantum Well Infrared Photodetector", Applied Physics Letters, Vol. 70, pp. 859-861 (1997) and a commonly assigned pending Application Ser. No. 08/928,292 filed on Sep. 12, 1997, which are incorporated herein by reference. Two different voltages are supplied to the detector to provide proper bias to different quantum-well detectors for substantially optimized responsivities. U.S. Pat. No. 5,552,603 to Stokes discloses a three-color quantum-well sensing array that requires two bias voltages for each sensing pixel.
This requirement of two different bias voltages presents a difficulty in forming a dual-band sensing array. A sensing array requires a multiplexer for readout but most commercial readout multiplexers can only provide a single bias voltage to the sensing pixels. It may be possible to design a special readout multiplexer capable of supplying two voltages. However, this increases the cost of the device. In addition, the need of two voltages complicates the circuitry.